The present invention relates to a magnetic recording medium having a ferromagnetic metallic thin film formed on a non-magnetic support, and in particular to a magnetic recording medium with a protecting layer containing a rust preventive and a lubricant on a ferromagnetic thin film, and also to a magnetic recording medium having a lubricating layer containing an organic fluorine compound on a magnetic layer.
The magnetic recording medium such as magnetic tape, floppy disk, etc., is divided to coating type magnetic recording medium, in which a coating solution containing ferromagnetic powder dispersed in a composition consisting of dispersing agent, bonding agent, etc. is coated on a non-magnetic support, and also to metallic thin film type magnetic recording medium, which comprises a ferromagnetic metallic thin film formed on a non-magnetic support by film forming method such as vacuum deposition, sputtering, ion plating, etc.
With the advent of high definition television system and technical progress such as digitization of recording system, there are now strong demands on magnetic recording medium suitable for high density recording.
The metal thin film type magnetic recording medium, which has a ferromagnetic thin film, made of ferromagnetic metal such as cobalt, nickel, etc. or alloy of these metals, as magnetic layer, is very suitable as the magnetic recording medium for high density recording because, unlike the coating type magnetic recording medium, it contains no material other than magnetic materials, such as bonding agent, in the magnetic layer.
However, in the metal thin film type magnetic recording medium, the surface of the metal thin film is susceptible to corrosion and such corrosion adversely affects electromagnetic transducing characteristics and has problems in preservation property and stability. Because the metal thin film is very thin and the surface is extremely smooth, it has serious disadvantage such as poor running property and low durability in practical application.
In particular, the magnetic recording medium is now used under a wide variety of environmental conditions, and there is imminent need to prevent corrosion of metal thin film under high temperature and high humidity conditions and to maintain preservation property under serious environmental conditions.
On the other hand, to maintain good running property and high durability, it is effective to form a lubricating layer containing fluorine compound on the surface of metal thin film. Above all, the compound having polar group is effective to provide stable lubricating protective layer because it firmly attaches on the surface of the magnetic layer. It is widely known that the organic fluorine compound containing perfluoroalkyl group is both hydrophobic and lipophobic and exhibits the property as surface active agent in water and oil. Also, it has property to reduce surface energy, and sliding property and lubricating property of materials can be improved by coating organic fluorine compound.
For example, Japanese Patent Publication Laid-Open 3-102614 describes a lubricant, which possesses --COCO-- and --SS-- at the center of perfluoro-polyether chain, while this causes corrosion problem on magnetic recording medium and thin film head because of --COCO-- and --SS--.
Japanese Patent Laid-Open Publication 64-72313 describes the formation of a monomolecular layer of perfluoroalkyl group - alkyl group - polar group on ferromagnetic metallic thin film. However, the formation of mono-molecular film by Langmuir-Blodgett's (LB) film is poor in productivity and is not very practical. Further, it is inferior in kinetic friction coefficient, .mu. value, still mode durability and preservation property.
In Japanese Patent Publication 3-19602 and Japanese Patent Laid-Open Publication 54-79366, a lubricant is disclosed, which contains fluorine type hydrocarbon compound having --SO.sub.3 Na, --SO.sub.3 K, --SO.sub.3 H, --COOH, etc., but this is also inferior in .mu. value, still durability, and preservation property.
Japanese Patent Laid-Open Publications 3-241524 and 62-92227 describe the coating of a compound having fluoroalkyl group and aliphatic alkyl group on ferromagnetic metallic thin film. Although this compound can be coated by isopropanol through the introduction of alkyl group, it is inferior in .mu. value, still durability and preservation property.
In addition to the coating of the above lubricant on magnetic recording layer, the use of perfluoroalkyl polyether and its derivatives is described (Japanese Patent Laid-Open Publications 60-61918, 61-107528, U.S. Pat. No. 3,778,308, Japanese Patent Publication 60-10368, U.S. Pat. No. 4,897,211, etc.). However, the lubricants containing fluorine type organic compound has high friction coefficient at low speed with the partner material when tape is sliding. The lubricants have also problems with preservation property because magnetic layer is detached due to corrosion by acidic gas such as sulfur dioxide and are not suitable as magnetic recording medium used in practical application.
Most of the above lubricants are not soluble in organic solvents of normal hydrocarbon type, such as acetone, methylethylketone, ethyl acetate, alcohols, toluene, etc., and it is necessary to use fluorine type organic solvents when coating. However, fluorine type organic solvents are expensive and also cause the problem of the destruction of ozone layer or aggravate the working environment and are not suitable for practical application.
Further, the problem of corrosion cannot be solved completely in case of fluorine compound, and it is not especially desirable to use the compound with polar group. The stronger the polarity is as in sulfonic group, the more it is not preferable to solve the corrosion problem.
As an approach to prevent rusting on ferromagnetic metallic thin film, there is a method to form oxide thin film on the ferromagnetic metallic thin film, while this reduces magnetic property of the film and also increases the distance between the head and the magnetic layer, thereby reducing electromagnetic transducing characteristics.
In this respect, various methods have been proposed to simultaneously use lubricant and rust preventive. For example, Japanese Patent Publication Laid-Open 61-82324 discloses a ferromagnetic metallic thin film type magnetic recording medium containing a compound having mercapto group as rust preventive, and compounds such as thiophenol, thiouracil, etc. are cited. It is also described to simultaneously use a fluorine type surface active agent as lubricant together with rust preventive. However, such rust preventive layers do not necessarily provide sufficient anti-rusting effect.
Japanese Patent Publication Laid-Open 64-82324 discloses formation of rust preventive layer of nitrogen-containing heterocyclic compound such as benzotriazole on the surface of a ferromagnetic metallic thin film, which contains cobalt as principal component. But, the rust preventive is not enough, and there is no description on synergetic effect with lubricant.
Japanese Patent Publication Laid-Open 58-194133 describes a magnetic recording medium, in which a rust preventive layer, containing a compound having a heterocycle with one or more nitrogen atoms and a phenolic hydroxide group, is formed on a ferromagnetic metallic thin film containing oxygen. As the rust preventives, pyridylazoresorcin, pyridyl-azonaphthol, quinolinol, quinolinediol, and kynurenic acid are disclosed, while these compounds are not enough to provide sufficient rust preventive effect and to ensure good running property and high durability.
Japanese Patent Publication Laid-Open 58-194135 discloses a rust preventive layer having a heterocycle with one or more nitrogen atoms to be attached on a ferromagnetic metallic thin film containing oxygen, but there is no description on simultaneous use with a compound having phenolic OH group or fluorine type lubricant.
Japanese Patent Publication Laid-Open 61-284822 describes a magnetic recording medium, in which an oxide film is formed on the surface of a ferromagnetic metallic thin film and a protective layer containing triazinedithiol is provided on it, but there is no description on rust preventive effect or simultaneous use with fluorine type lubricant.
Also, Japanese Patent Publication Laid-Open 58-189835 describes an adsorption layer of rust preventive agent formed on a ferromagnetic metallic thin film and a lubricant layer formed on the adsorption layer, and rust preventive such as alkylamine, alkyl phosphate, divalent phenol, diarylketone, alkylphenol, naphthol, quinone, nitroso compound, etc. are disclosed, while these rust preventives cannot provide satisfactory rust preventive effect. As the lubricant, fatty acid, fatty acid ester, fatty acid amide, perfluoroalkyl carboxylic acid and sulfonic acid, or fluorine type surface active agent such as perfluoroalkyl fatty acid ester, etc. are disclosed, but sufficient anti-friction effect cannot be obtained.
In case rust preventive and fluorine type lubricant are simultaneously used, the functions of each compound cannot be perfectly fulfilled probably because these compounds inhibit adsorption on metal thin film against each other.
As described above, no effective means has been proposed, which can solve the problems of metal thin film type magnetic recording medium for high density recording, i.e. the problems to improve or increase running property and durability and to improve preservation property and stability.
To solve the above problems, it is an object of the present invention to provide a magnetic recording medium with high density recording, which can provide higher running property, durability and attain high density magnetic recording characteristics and compact design of the metal thin film type medium.